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Ben Braun, MD, PhD

Background: Cancer arises when mutations subvert the mechanisms used by cells to control proliferation, differentiation and survival. Curing cancer with signal transduction inhibitors is possible if, and only if, a specific catalytic activity is essential for survival of cancer cells but non-essential for the rest of the organism. Our knowledge of biochemistry in primary cancer cells is too limited at present for this strategy to work well. Furthermore, it is particularly important to understand the biochemistry of long-lived tumor cells, even when such “cancer stem cells” make up a small fraction of the overall tumor bulk.

The Ras signaling network is activated by nearly all growth stimuli, and it is a frequent target of oncogenic mutations that lead to its inappropriate activation. About one-third of human cancers have mutations in one of the three RAS genes, KRAS, NRAS, or HRAS. We primarily study myeloid leukemia because there is a high incidence of KRAS and NRAS mutations and the hematopoietic system is uniquely accessible for studying both cell fates and biochemistry. We have used a conditional knock-in Kras allele to produce a model of human leukemia that is highly accurate at both genetic and phenotypic levels.

Major Goals: (i) to define the influence of Ras signaling on the fate of normal and malignant hematopoietic stem cells; (ii) to discover biochemical mechanisms underlying the effects of Ras in hematopoiesis; (iii) to find new opportunities for treating Ras-related cancers based on their unique biochemical regulation of cell survival and proliferation.

On-going Research:

Ras Oncogenes in Leukemia Stem Cells: Ras mutations cooperate with a variety of other lesions to cause a fully transformed malignancy. However, oncogenic Ras has profound effects by itself on the biology of hematopoietic stem cells. Mice with the conditional knock-in Kras allele die from an aggressive ‘myeloproliferative disease,’ in which excessive production of mature and immature hematopoietic cells leads to death. Similar diseases in people are associated with RASmutations. We have found that mutant Kras does not immortalize differentiated cells. By contrast,Kras mutant hematopoietic stem cells proliferate excessively and give rise to a durable and fatal disease. Our ongoing work aims to delineate the mechanism of cell cycle dysregulation in Krasmutant hematopoietic stem cells, and to better understand how Ras signaling influences stem cell fate. We are also using these mice to test signal transduction inhibitors as potential new therapies for leukemia.

Anemia due to excessive Ras signaling: Despite causing a growth advantage in stem cells, mutant Ras has a negative effect in red cell precursors. This leads to a profound anemia in Krasmutant mice. We are examining both cell-intrinsic and -extrinsic processes in Kras mutant mice to investigate the fate of these progenitor cells.

SHP-2 Signaling in Leukemia: SHP-2 is a cytoplasmic protein tyrosine phosphatase encoded by the gene PTPN11. In general, SHP-2 promotes signaling through the Ras and MAPK systems. As for Ras, oncogenic mutations in PTPN11 result in excessive SHP-2 activity. However, the precise mechanism by which SHP-2 modulates signal transduction is unknown. Using a genetic approach, we have found that SHP-2 has at least one important function upstream of Ras, and we are currently investigating potential substrates that might mediate this effect. We have also discovered a novel role of SHP-2 that is independent of its catalytic activity. Because SHP-2 has multiple protein interaction domains, it is likely it can also serve as an adaptor molecule. We are also looking for the relevant binding partners using proteomic methods.

Modeling Embryonal Rhabdomyosarcoma: Embryonal rhabdomyosarcoma is an aggressive tumor of early childhood that arises in skeletal muscle. While often curable, survivors undergo intensive chemotherapy and endure tremendous morbidity from surgery and/or high dose radiation therapy. These tumors are genetically complex, but approximately 25% exhibit oncogenic Ras mutations (H- K- or N-Ras). Other common lesions include loss of imprinting at 11p15, p16INK4A/CDKN2A loss and p53 disruption. Some of these loci are also implicated by genetic syndromes associated with high rates of embryonal rhabdomyosarcoma: Costello syndrome (HRAS), Beckwith-Wiedemann syndrome (11p15), and Li-Fraumini syndrome (p53). We are building a murine model of embryonal rhabdomyosarcoma based on expression of oncogenic Ras in developing muscle to further investigate how oncogenes cooperate in this disease. We intend to use the experience we have gained through studying Ras in the hematopoietic system to effectively interrogate effects on biochemistry and cell fate imparted by mutant Ras in sarcoma.